A compact disc player generates a light beam which is focused onto the recording medium by means of a focus regulation circuit, and which is guided along the data tracks of the disc by a tracking regulation circuit. Tracking regulation circuits typically include a coarse drive circuit for energizing apparatus to effect coarse or large radial displacements of the light beam, and a fine drive circuit for energizing apparatus to effect fine or precise radial displacements of the light beam. The coarse drive apparatus may, for example, be a motor which drives a spindle to translocate an optical scanning assembly. The optical scanning assembly may include several lenses, a prismatic beam-splitter and a photodetector, compliantly suspended and arranged for movement as a galvanometer. The galvanometer like arrangement effects at least the fine radial displacement.
Using the fine drive, the light beam can be moved by approx. .+-.0.5 mm in the radial direction which translates to about .+-.300 data tracks. Since this distance relates to only a very small percentage of the total number of tracks contained on a disc, the coarse drive must be energized from time to time, during normal playback, to provide continuous radial tracking of the light beam.
One disadvantage of a tracking regulation circuit with a coarse and a fine drive lies in the fact that the coarse drive may generate mechanical vibrations when energized. These vibrations will be mechanically transmitted to the optical scanning device. The vibrations are actually compensated for by the focus regulation circuit and the fine drive so that the scanning of the data is not interrupted through loss of focus or data tracking. If, however, while the coarse drive is operating, the compact disc player is subjected to external shocks or impacts, such as frequently occur in compact disc players installed in a vehicle, the vibrations caused by the coarse drive and the external shocks can reinforce each other. The consequences of this may be a tracking loss. In fact, under unfavorable conditions, the focus regulation circuit will no longer be able to focus the light beam onto the disc.
Disc faults such as scratches or fingerprints may also lead to tracking loss and to loss of focus if they occur while the coarse drive is running.
It is, therefore, the object of the invention to achieve reliable scanning of data with a recording and/or playback apparatus employing contact-less scanning of a recording medium, particularly in the case of external shocks and/or obstructions on, or defects in, the recording medium.